Odontostomatous (a.)
definition: Having toothlike mandibles; -- applied to certain insects.

Odontostomatous is a 15 letter word that begins with an O and ends in an S

Old-gentlemanly (a.)
definition: Pertaining to an old gentleman
Or like one.
Old-gentlemanly is a 15 letter word that begins with an O and ends in a Y

Omnipercipience (n.)
definition: Alt. of Omnipercipiency

Omnipercipience is a 15 letter word that begins with an O and ends in an E

Omnipercipiency (n.)
definition: Perception of everything.

Omnipercipiency is a 15 letter word that begins with an O and ends in a Y

Omnium-gatherum (n.)
definition: A miscellaneous collection of things or persons; a confused mixture; a medley.

Omnium-gatherum is a 15 letter word that begins with an O and ends in an M

Omphalomesaraic (a.)
definition: Omphalomesenteric.

Omphalomesaraic is a 15 letter word that begins with an O and ends in a C

Omphalopsychite (n.)
definition: A name of the Hesychasts
From their habit of gazing upon the navel.
Omphalopsychite is a 15 letter word that begins with an O and ends in an E

Oneirocriticism (n.)
definition: Alt. of Oneirocritics

Oneirocriticism is a 15 letter word that begins with an O and ends in an M

Ophthalmologist (n.)
definition: One skilled in ophthalmology; an oculist.

Ophthalmologist is a 15 letter word that begins with an O and ends in a T

Opisthobranchia (n.)
definition: Alt. of Opisthobranchiata

Opisthobranchia is a 15 letter word that begins with an O and ends in an A

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